The McGurk Effect is a fascinating auditory illusion where you can be fooled by what you are seeing into hearing something different. A remarkable property of the McGurk Effect is that even though you know you are being fooled, you still hear the wrong thing!
What’s going on here is that when the brain tries to process a complex sound – and virtually all interesting sounds are indeed very complex – it tries first to create a model for what it thinks it is going to hear, and adapts what it actually hears to best match with the model. Since humans are primarily visually-driven beings, the brain will above all else try to match what we hear to what we see. This has some profound implications for listening to high-end audio, and understanding what it is you think you can hear.